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Bureau of Labor Statistics > Consumer Price Index > Home

CPI Addendum to Frequently Asked Questions

The CPI is the most widely used measure of inflation and is sometimes viewed as an indicator of the effectiveness of government economic policy. It provides information about price changes in the Nation's economy to government, business, labor, and private citizens and is used by them as a guide to making economic decisions. In addition, the President, Congress, and the Federal Reserve Board use trends in the CPI to aid in formulating fiscal and monetary policies.
The CPI and its components are used to adjust other economic series for price changes and to translate these series into inflation-free dollars. Examples of series adjusted by the CPI include retail sales, hourly and weekly earnings, and components of the National Income and Product Accounts.

An interesting example is the use of the CPI as a deflator of the value of the consumer's dollar to find its purchasing power. The purchasing power of the consumer's dollar measures the change in the value to the consumer of goods and services that a dollar will buy at different dates. In other words, as prices increase, the purchasing power of the consumer's dollar declines.
The CPI is often used to adjust consumers' income payments (for example, Social Security) to adjust income eligibility levels for government assistance and to automatically provide cost-of-living wage adjustments to millions of American workers. As a result of statutory action the CPI affects the income of millions of Americans. Over 50 million Social Security beneficiaries, and military and Federal Civil Service retirees, have cost-of-living adjustments tied to the CPI. In addition, eligibility criteria for millions of food stamp recipients, and children who eat lunch at school, are affected by changes in the CPI. Many collective bargaining agreements also tie wage increases to the CPI.

Another example of how dollar values may be adjusted is the use of the CPI to adjust the Federal income tax structure. These adjustments prevent inflation-induced increases in tax rates, an effect called bracket creep.
Traditionally, the CPI was considered an upper bound on a cost-of-living index in that the CPI did not reflect the changes in buying or consumption patterns that consumers would make to adjust to relative price changes. The ability to substitute means that the increase in the cost to consumers of maintaining their level of well-being tends to be somewhat less than the increase in the cost of the mix of goods and services they previously purchased.

Since January 1999, a geometric mean formula has been used to calculate most basic indexes within the CPI; in other words, the prices within most item categories (for example, apples) are averaged with the use of a geometric mean formula. This improvement moves the CPI closer to a cost-of-living measure, because the geometric mean formula allows for a modest amount of consumer substitution as relative prices within item categories change.

It is important to note that area CPIs cannot be used to compare levels of living costs or prices across areas. (See answer to Question 18: "Can the CPIs for individual areas be used to compare living costs among areas?")
For example, if you or your family spends a larger-than-average share of your budget on medical expenses, and medical care costs are increasing more rapidly than the cost of other items in the CPI market basket, your personal rate of inflation may exceed the increase in the CPI. Conversely, if you heat your home with solar energy, and fuel prices are rising more rapidly than other items, you may experience less inflation than the general population does. A national average reflects all the ups and downs of millions of individual price experiences. It seldom mirrors a particular consumer's experience.

This information enabled BLS to construct the CPI market basket of goods and services and to assign each item in the market basket a weight, or importance, based on total family expenditures. The final stage in the sampling process is the selection of the specific detailed item to be priced in each outlet. This is done in the field, using a method called disaggregation. For example, BLS economic assistants may be directed to price "fresh whole milk." Through the disaggregation process, the economic assistant selects the specific kind of fresh whole milk that will be priced in the outlet over time. By this process, each kind of whole milk is assigned a probability of selection, or weight, based on the amount the store sells. If, for example, Vitamin D homogenized milk in half-gallon containers makes up 70 percent of the sales of whole milk, and the same milk in quart containers accounts for 10 percent of all whole-milk sales, then the half-gallon container will be 7 times as likely to be chosen as the quart container. After probabilities are assigned, one type, brand, and container size of milk is chosen by an objective selection process based on the theory of random sampling. The particular kind of milk that is selected by disaggregation will continue to be priced each month in the same outlet.

In sum, price changes are weighted by the importance of the item in the spending patterns of the appropriate population group. The combination of all these factors gives a weighted measurement of price change for all items in all outlets, in all areas priced for the CPI.
The following are some examples of technical or statistical guidelines from BLS:
- For escalation, BLS strongly recommends using indexes that are not seasonally adjusted. (See answer to Question 16, for a further explanation of seasonally adjusted indexes and the reasons BLS does not recommend seasonally adjusted indexes for use in escalation.)
- Also for escalation, BLS recommends using national or regional indexes, due to the volatility of local indexes. (See answer to Question 17, for an explanation of this point.)
  For those with further questions, BLS has prepared a fact sheet, Using the Consumer Price Index for Escalation (PDF). This information also may be obtained by writing or calling the nearest BLS regional office listed in the answer to Question 23. You may also call the BLS national office at (202)691-700.
The following illustration shows that although Area B has higher prices than Area A, the price change in Area A has been greater than in Area B.
```
                  Base Period               Current Period
                  Price    Index             Price    Index
    Area A        $0.30     100               0.55     183
    Area B         0.60     100                .90     150
```

The CPI thus measures the rates of change in prices, rather than the level of prices.

Last Modified Date: March 2, 2011